2 Innovative Techniques for 3d Digital Survey of the Paphos Theatre Paola RONZINO, Italy 1. INTRODUCTION Nowadays the use of digital acquisition techniques is widespread in archaeology and in the cultural heritage field, mainly applied for documentation and conservation issues. This project aims at producing a 3D model of the Hellenistic-Roman Theatre of Paphos in Cyprus, by integrating two different acquisition technologies such as aerial photogrammetry and laser scanning, for documentation, dissemination and for reconstruction purposes and for the test of different technologies involved in the project. This work is a collaboration between the Science and Technology for Archaeology Research Center (STARC) of the Cyprus Institute (CY), the Institute for Technology Applications to Cultural Heritage of the National Research Council (CNR-ITABC) Italy, the University of Naples L Orientale (CISA), Italy and the University of Sydney-Archaeological mission in Paphos. The result of the work consists in a 3D model suitable for many uses. It can be viewed and processed with appropriate software, processed with CAD software, visualized in different environments such as Internet, standard monitors, special 3D visualization monitors and so on (N. AMICO et al 2010). The pipeline followed for data acquisition and data processing has an experimental character due to the originality of the adopted instruments (automatic photogrammetry) and because it was never tested before in similar conditions. The outcome gave a satisfactory result with a high level of accuracy and a perfect overlap between the models achieved with the two different techniques. 2. CASE STUDY: THE PAPHOS THEATRE The site of the ancient theatre of Nea Paphos is located in the modern town of Kato Paphos. It was built on the southern slope of a hill, which is in the very north-east part of the ancient walled city. It measured m from side to side and had a seating capacity for over 8000 spectators. The angle of rise of the seating is 26.5 degrees. The theatre has been built around 300 BC. It is possible to identify at least five major phases of remodelling and rennovation during the its history representing the changing nature of performance during Greek and Roman audiences, and the responses to earthquake damage. At its peak, in the mid-second century AD under the Roman Antonine emperors, when the stage building was façaded in marble, the theatre measured over 90 meters from side to side, and had a seating capacity of over 8000 spectactors. By the end of the third century AD, probably after the devastating earthquake of 365 AD, the theatre was abandoned and much of the stonework was robbed and later quarried for use elsewhere in the town. After a period of abandonment, the site of the ancient theatre sees renewed activities in the 12th and 13th centuries AD, when the harbour of Paphos became 2/9

3 once again a major economic point of activity, this time for the Crusaders on their way to the Holy Land (BARKER G et al 2004). 3. ACQUISITION AND POST PROCESSING For the digital acquisition of the theatre two innovative techniques were used: aerial photogrammetry with an aerostatic balloon, for the acquisition of the whole extension of the theatre; and laser scanning for the acquisition of the part difficult to reach from above. ZF IMAGER 5003 Fly scan Technology Phase shift Automatic Photogrammetry Range of acquisition m Up to 100 mt above the ground Scan rate Up to p/s Undefined number of shots Field of view 360 x Accuracy 1 mm 1-2 cm Integrated camera no Set of three cameras Software JRC 3D Reconstructor Z-Scan; Z- Map Tab.1: Specification of the instruments 3.1 Aerial photogrammetry The automatic photogrammetry, is an experimental technique that concerns the acquisition of a sequence of shots from meters above the ground. In this case the height was 40 m. The pictures were taken from a set of three cameras (Nikon D80), with 24 mm lens, previously calibrated and lined up, suspended to a balloon with two aluminium (T shaped) bars 2.20 m long. A remote controller was connected for the shooting of the photos. Fig.1: The Fly scan system during data acquisition. 3/9

4 The system (Fly scan) is an innovative device implemented by the company Menci Software, Arezzo, in collaboration with the Institute for Technology Applications to Cultural Heritage of the National Research Council (CNR-ITABC). The diameter of the balloon is about 4,7 m. It is tied to the ground with ropes hold by four operators who drive the system. The balloon is filled with helium. Beside the pictures from above, two additional sequences of shots were taken from the ground with the same set of three cameras. In the first sequence the cameras were positioned on the bar with the focal at the zenith and in the second sequence with the focal skewed at 45 degrees, in order to acquire the front elevation of the theatre. More over, in this project, it has been tested a new technolgy close range photogrammetry for the acquisition of structures of short distance. Fig.2 : Close range photogrammetry. Previously, a set of targets was laid on the entire surface of the theatre and acquired with the total station Trimble The point set was then geo-referenced using a Differential GPS system, in order to enable the assembling of the sequence of pictures. The targets were A4 black paper with white cross (to reduce the sun glare) visible also from above. The triplets of pictures generated with each shot were elaborated with a dedicated software (Z-Scan) for the generation of the point clouds and for a first elaboration of the threedimensional data 1. A second software (Z-Map) was used for cleaning, filtering, texturing and merging of point clouds and for the creation of orthophotos, cross sections and maps. The outcome is a textured color per vertex model. 1 The software uses an algorithm that identifies the pixels with the same colorimetric characteristics creating a point cloud. 4/9

5 Fig.3: Point cloud of the whole area of the Theater from aerial photogrammetry. 3.2 Laser scanning The survey described above was integrated by means of the digital acquisition of some parts of the theatre with a phase shift 3D scanner (Z-F IMAGER 5003). Fig. 4: Laser Scanner Z-F IMAGER 5003 Twenty-five scans (ten for the summa cavea and fiftheen for the parodos) were necessary to gather all the details of interiors parts and the vertical face of the theatre not visible from the top. Targets were laid also in these areas in order to help the alignment of the scan range. 5/9

6 Two days were necessary for the acquisition on the field and one week in the laboratory for scans cleaning with the application of various filters. The single meshes were than merged together and aligned. During the acquisition process with Z-F IMAGER 5003 scanner, data is acquired both in spatial coordinates x,y,z, and in the reflectance value (tones of gray that represents the response of the material to the laser beam). In order to assign the texture to the point clouds, another step is necessary. A set of picture was taken with a resolution of 3488x2616 pixels and then they were applied to the point cloud using the ground control point (GCP) as reference target. The software used was JRC 3D Reconstructor that enables to export the outcomes in all standard format file. 4. INTEGRATION PROCESS The next step involved the integration of the outcomes. Using the ground control point, the point cloud generated from aerial photogrammetry and the scans from the laser scanner were aligned, with an error of 6 mm. The final model has been georeferenced to the same absolute coordinate system, according to 3 points measured with a Differential GPS. Fig.5: Integration of the point cloud from aeria photogrammetry (left) with a point cloud from laser scanner (right). 5. CONCLUSIONS The final result is a model generated from the integration of the two techniques (photogrammetry and laser scanner). It is satisfactory because the different parts fit with an high accuracy (error less than 6 mm). Each of the two tecniques has of course pros and cons. For example the Fly Scan System is less expensive than a laser scanner, but has a stronger reliance on atmospheric conditions: e.g wind, sun light (a strong sun light causes the lost of a lot of informations). These factors does not affect the acquisition with the laser scanning. 6/9

7 With the aerial photogrammetry it is possible to gather a large amount of data without having the problem of the laser scanning, for which the size of the outcomes is too heavy to be managed. The aerial photogrammetry guarantees a faster acquisition process in the field (in this case one day for the acquisition of the whole area) compared with the laser scanning (two days for twenty-five scans) and an easy processing of the photos with intuitive software (one week), giving the possibility to modify the resolution of the point clouds during the processing phase according to the final use. On the other hand, the first phase of the Fly Scan set up takes a longer time than setting a laser scanning. Another constraint is the availability of helium to fill the baloon, not easily available everywhere. The outcomes suggest to integrate the two techniques in order to achieve a complete and accurate documentation of the object of study, using photogrammetry for large scale acquisition, but with lower resolution (1-2 cm), and laser scanning for the acquisition of fine details with higher resolution (1 mm). The research goal of the work and the expected results are in sum the key factors to decide which tecnologies to use. Fig. 6: The final integrated 3D model. ACKNOWLEDGEMENTS The survey was carried out in April 2009 by the CNR-ITABC team (Nicola D Amico and Andrea Angelini) led by Dr. Roberto Gabrielli and the CISA team (Giancarlo Iannone) led by Dr. Andrea D Andrea. The STARC team included the author of the present paper. This research in partially supported by the European Commission under the 7th Framework Programme throught the 3D-COFORM project, grant /9

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